Novel Plasmonic Material Discovered

Titanium nitride, the first nonmetal to be added to the short list of surface-plasmon-supporting materials, could bridge the gap between optics and electronics. The advance could point the way to a new class of optoelectronic devices with unprecedented speed and efficiency.

Until now, the best candidates for plasmonic materials were gold and silver. However, these noble metals are not compatible with standard silicon manufacturing technologies, limiting their use in commercial products. Silver also degrades when exposed to air, causing a loss of optical signal and making it a less attractive choice for plasmon technology.

Now researchers at Purdue University are studying the plasmonic capabilities of titanium nitride, a ceramic material used to coat metal surfaces. Titanium nitride was chosen as a test material because it is easy to manipulate in manufacturing, and it can be easily integrated into silicon semiconductor devices. Titanium nitride can also be grown one crystal at a time, allowing it to form highly uniform, ultrathin films.

a) Excitation by light of a surface plasmon-polariton on a thin film of titanium nitride. b) Atomic force microscope image of the surface of titanium nitride film. c) Scanning electron microscopy image of titanium nitride thin film on sapphire. (Image: Alexandra Boltasseva, Purdue University/Optical Materials Express)
“Titanium nitride is a promising candidate in the near-infrared and visible wavelength ranges. Unlike gold and silver, titanium nitride is compatible with standard semiconductor manufacturing technology and provides many advantages in its nanofabrication and integration,” said Alexandra Boltasseva, the lead researcher on this experiment.

Boltasseva's team laid a very thin film of titanium nitride evenly over the surface of a sapphire to measure its plasmonic capabilities. The group found that titanium nitride transmits plasmons about as well as gold does, but not as efficiently as silver under ideal conditions. The researchers are now seeking to improve the performance of titanium nitride using a manufacturing technique called molecular beam epitaxy, which enables the crystal-by-crystal growth of superlattices.

They believe that titanium nitride could outperform the noble metals in certain metamaterial and transformation optics devices such as, those based on hyperbolic metamaterials.

“We have found that titanium nitride is a promising candidate for an entirely new class of technologies based on plasmonics and metamaterials,” said Boltasseva. "This is particularly compelling because surface plasmons resolve a basic mismatch between wavelength-scale optical devices and the much smaller components of integrated electronic circuits.”